Producing renewable clean energy is one of the most profound challenges of the 21st century. Most of the world's current energy supplies come from sunlight converted to chemical energy by plant photosynthesis. Photosynthesis begins with light striking a light harvesting complex in Photosystem II, creating a charge-separated excited state where an electron is promoted to a higher energy level. Water oxidation catalyzed by an oxygen evolving complex (OEC) replenishes the hole derived from the charge-separated excited state. This process causes the release of four equivalents of H+ per water molecule. Some of the generated protons are used in a proton gradient, which the plant employs to store energy by synthesizing ATP. The high energy electrons move along an electron transport chain and are eventually used to reduce the remaining protons from the water oxidation process.
A central thrust of current energy research focuses on artificial photosynthesis, namely synthetic light-harvesting, water-based fuel producing systems. Despite the intense global efforts to develop viable a biological water splitting systems for energy production, breakthroughs are needed in selectivity, speed, and stability of the three operational units in the catalyst system: the sensitizer for light absorption, the catalyst for water reduction, and the catalyst for water oxidation.
Developing a synthetic viable water oxidation catalyst (WOC) has proven particularly challenging. An effective WOC must be fast, capable of water oxidation at a potential minimally above the thermodynamic value (H2O→O2+4H+ +4e−; 1.23 V), and, stable to air, water and heat (i.e., oxidative, hydrolytic, and thermal stability). Both heterogeneous and homogeneous WOCs have been investigated. Heterogeneous WOCs generally have the advantages of low cost, ease of interface with electrode systems, and oxidative stability, but they are harder to study and thus are harder to optimize compared to homogeneous catalysts. Further, heterogeneous catalysts can be deactivated by surface poisoning or aggregation.
In contrast, homogeneous WOCs are more amenable to spectroscopic, crystallographic, physiochemical, and computational investigation, and thus can be readily optimized. Additionally, each individual molecule of a homogeneous catalyst is capable of doing chemistry, a cost issue when precious metals such as Ruthenium are involved. However, many of the known homogeneous catalysts that have been investigated contain organic ligands that are thermodynamically unstable with respect to oxidative degradation. As a result, all homogenous WOCs with organic ligands reported to date are oxidatively deactivated.
There exists a need for WOCs that have the stability, durability, and accessibility of heterogeneous metal oxide catalysts with the activity, selectivity and tunability of homogeneous catalysts.
Therefore, it is an object of the invention to provide hydrolytically, oxidatively, and thermally stable WOCs for the rapid oxidation of water and methods of making and using thereof.
It is a further object of the invention to provide WOCs prepared from inexpensive and abundant metals.
It is still further an object of the invention to provide stable WOCs prepared from inexpensive and abundant metals that exhibit improved turn over number, turn over frequency, and/or oxygen yield.